The “Other” Double-Double in Lyra: Struve (Σ) 2470 and Struve (Σ) 2474

Struve (Σ) 2470        HIP: 94043     SAO: 67870
RA: 19h 8.8m   Dec: +34° 46′
Magnitudes:  7.03, 8.44
Separation:   13.9″
Position Angle: 267°   (WDS 2013)
Spectral Type: B5
Distance: 1320 Light Years
Status:  Not determined

Struve (Σ) 2474     HIP: 94076     SAO: 67879
RA: 19h 9.1m   Dec: +34° 36′
Magnitudes  AB: 6.78, 7.88     AC: 6.78, 11.42
Separation   AB: 15.9″            AC: 96.80″
Position Angle   AB: 263° (WDS 2013)    AC: 123° (1998)
Spectral Type: “A” is G1, “B” is G5
Distance: 160 Light Years
Status:  Physically related (WDS, codes U & Z);  AC is WAL 105

(WDS data updated 10/6/2014)

This system of stars has earned a limited amount of fame as the “other” double-double in Lyra.  Located to the south and east of Epsilon (ε) Lyrae, the better known system of that name, these four stars would very likely get more attention if they were located in a constellation with fewer objects to demand your attention — such as Ursa Minor ……………

Lyra, showing Σ 2470 and Σ 2474 to the east. These two stars from a triangle with Delta (δ) and Gamma (γ) Lyrae — or, you can locate them by landing halfway between Iota (ι) and 17 Lyrae. (Stellarium screen image with labels added, click on the image for a larger view)

…………….  which is where I had planned to be on this night.  But plans are never seriously made with intentions of following them, at least not when searching the sky for 60mm delights.

In a 60mm f16.7 with a 20mm Plössl, giving me 50x, I could see four stars immediately.  But what really grabs your attention is the orientation of  each of these pairs — they’re parallel to each other!   And symmetrical as well!  In my 60mm, each one of the pairs consists of a bright component with a fainter one hanging on for dear life at the edge.

Σ 2470 is shown at the top left, and Σ 2474 is to the right and below it. Also shown is the “C” component of Σ 2474, which goes by the name of WAL 105 in the WDS. East and west are reversed to match the view in a refractor, click for a larger view. (STScI photo)

Really an unusual sight, and much more deserving of attention than they get.  (In case you’re wondering, the pairs of Epsilon (ε) Lyrae are oriented at roughly a ninety degree angle).  To my eyes, all four of these are white in color, but Haas sees some yellow in Struve 2474.

This is a sketch done by Steve McGee at the summit of Haleakala in Hawaii. East & west are reversed here to match the SCT view, click on the sketch for an eye-pleasing larger version.   For those who prefer an uncluttered view, I included an un-labeled version at the end of the post.

There’s more than meets the eye, though, with this pair of stars.  Σ 2470 is a spectroscopic binary, but it’s the other one Σ 2474, that’s really complicated.  It’s primary consists of two stars lying one-tenth of an arc second apart.  If you have a pressing urge to find them, the Washington Double Star Catalog (WDS) statistics for 2008 show the secondary (Ab) at a magnitude of 8.96 with a position angle of 258 degrees — but don’t get your hopes up too much unless you’re sitting at the eyepiece of a hundred inches or so of aperture!  That pair of stars carries the designation CHR 84.

But hang on — there’s yet another star listed in the WDS, and this one lies within reach of most telescopes.  Identified as WAL 105, it’s the “C” component of Σ 2474.  It sits at a distance of 96.8″ with a position angle of 123 degrees, and a moderate magnitude of 11.4 — those figures were updated in the WDS in 1998 — and it’s labeled on the photo above.   First identified in 1940, by 1998 it was determined that it’s not physically related to either Σ 2474 “A” or “B.”

The uncluttered view! Many thanks to Steve for the use of the sketch.